really this depends on whether he is listening to radio, (which can easily be jammed, or broadcast over ;)) , or if he's listening to CDs, MP3s, vinyl etc, then you will need to "affect" his amplifier, which is obviously more difficult.
a simple solution could be to get a hold of a remote for the stereo he has, using some kind of reflector between your and his windows.... could work, depending on the position of his stereo with his window